In the manufacture and processing of planar material webs, such as paper webs and plastic and metal sheeting, the material webs are guided and transported between rotating rollers. To precisely guide and align the webs, a control loop is used in which the lateral position of the material web constitutes the control variable.
An optical sensor or an ultrasonic sensor continuously detects the position of a longitudinally extending feature, typically the side edge of the material web or an imprinted marker line. From the detected position of the marking, a web travel controller calculates the offset of the web from a specified target position and, if required, drives a pivoted frame forming the actuator such that the material web is laterally guided back to its target position.
Optical line sensors for controlling a material web at a printed line or a material edge are known. In such an optical line sensor, a light transmitter in the sensor generates a light spot on the surface of the material to be scanned. The reflected light is recorded by two triangular photodiodes and analyzed in an analog circuit. Prior to operation, the line sensor must be set to the color contrast of the material to be scanned. To do this, the sensor is manually positioned on a line imprinted on the material web and a certain key sequence is input into the electronics. Then the sensor is directed at the background and a further key sequence is input into the electronics. The sensor is then ready for operation and correctly set. Such sensors are reliable and sturdy, but there are restrictions on the admissible line width of the printed lines, as well as in recognizing different contrasts.
U.S. Pat. No. 6,566,670 B1 describes a system for guiding a web of moving material that includes as illumination sources four multicolored light-emitting diodes that are disposed at different positions relative to the focus point on the material web. To operate the system, an operator selects, according to the reflection properties of the material web (matte, glossy or holographic), the first (for matte webs), the second (for glossy webs) or the third and fourth light-emitting diode together (for webs with holographic properties). Further, the color of the selected light-emitting diode (red, blue, green or infrared) is set depending on the type of material web and the color of the line to be tracked. A disadvantage of this system is that the operator's eye recognizes colors, while the photodiodes of the sensor detect only gray scale values, so that an optimal setting is often difficult to carry out, and is achieved only after several setting procedures by trial and error. Therefore, a new and improved device and method for detecting a feature of a running material web are needed.